The heat shock (HS) response is the major cellular defense mechanism against acute exposure to environmental stresses. The hallmark of the HS response, which is conserved in all eukaryotes, is the rapid and massive induction of expression of a set of cytoprotective genes. Most of the induction occurs at the level of transcription. The master regulator, heat shock transcription factor (HSF, or HSF1 in vertebrates), is responsible for the induction of HS gene transcription in response to elevated temperature (Voellmy, EXS, 1996, 77:121-137, Morimoto et al., EXS, 1996, 77:139-163).
It has been recently disclovered that HSF activation by heat shock is mediated by a ribonucleoprotein ternary complex comprising translation elongation factor eEF1A and a Heat Shock RNA (HSR1), a highly evolutionary concerved non-coding RNA (Shamovsky et al., Nature, 2006, 440:556-560). Among the two HSF-associated factors, HSR1 serves as a cellular thermosensor that determines the temperature threshold for the heat shock response. HSR1 and eEF1A are both required for activation of HSF and constitute a minimal functional HSR1 activating complex.
Under normal conditions HSF is present in the cell as an inactive monomer. During HS, HSF trimerizes and binds to a consensus sequence in the promoter of HS genes, stimulating their transcription by up to 200-fold. Most of these genes encode heat shock proteins (HSPs), a large family of molecular chaperones, which includes several functional and molecular weight sub-families. HSPs are essential for cell survival under normal conditions and are critical for cell survival during stress (see, e.g., Ellis, Trends Biochem Sci., 2000, 25: 210-212; Forreiter and Nover, J. Biosci., 1998, 23: 287-302; Hartl and Hayer-Hartl, Science, 2002, 295: 1852-1858; Haslbeck, Cell Mol. Life Sci., 2002, 59: 1649-1657; Young et al, Trends Biochem. Sci., 2003, 28: 541-547; Soll and Schleiff, Nature Rev. Mol. Cell. Biol., 2004, 5: 198-208). HSPs are considered a part of a protective mechanism against certain pathological conditions, including ischemic damage, neurodegenerative diseases, ageing, infection, and inflammation (Klettner, Drug News Perspect. 2004; 17:299-306; Hargitai et al., Biochem. Biophys. Res. Commun. 2003; 307:689-695; Yenari et al., Ann. Neurol. 1998; 44:584-591; Suzuki et al., J. Mol. Cell. Cardiol. 1998; 6:1129-1136; Warrik et al., Nat. Genet. 1999; 23:425-428; Pockley, Circulation 2002; 105:1012-1017; Hsu et al., Science 2003; 300:1142-5; Morley and Morimoto, Mol. Biol. Cell 2004; 15:657-64; Westerheide and Morimoto, J. Biol. Chem. 2005, 280:33097-100; Batulan et al., J. Neuosci. 2003; 23:5789-5798; Guzhova et al., Brain Res. 2001; 914:66-73; Wyttenbach et al., Human Mol. Gen. 2002; 11:1137-51; Warrick et al., Nat. Genet. 1999; 23:425-8; Krobitsch and Lindquist, Proc. Natl. Acad. Sci. USA 2000; 97:1589-94).
In the case of inflammation, a protective role of HSPs has been shown in a variety of experimental models (Jattela et al., EMBO J. 1992; 11:3507-3512; Morris et al., Int. Biochem. Cell Biol. 1995; 27:109-122; Ianaro et al., FEBS Lett. 2001; 499:239-244; Van Molle et al., Immunity 2002; 16:685-695; Plumier et al., J. Clin. Invest. 1995; 95:1854-1860; Marber et al., ibid., pp. 1446-1456; Radford et al., Proc. Natl. Acad. Sci. USA, 1996; 93:2339-2342). For example, Ianaro et al. (Mol. Pharmacol. 2003; 64:85-93) have recently demonstrated that HSF1-induced HSP72 expression in the inflamed tissues and activation of the heat shock response is closely associated with the remission of the inflammatory reaction. It follows, that HSP genes may function as anti-inflammatory or “therapeutic” genes, and their selective in vivo transactivation may lead to remission of the inflammatory reaction (Ianaro et al., FEBS Lett. 2001; 499:239-244 and Ianaro et al., FEBS Lett. 2001; 508:61-66).
Heat shock is also a known transcriptional activator of human immunodeficiency virus type 1 (HIV) long terminal repeat (LTR). However, HIV LTR suppression can occur under hyperthermic conditions (Gerner et al., Int. J. Hyperthermia 2000; 16:171-181). Indeed, the inhibition of HIV transcription has been reported after whole-body hyperthermia at 42° C. in persons with AIDS (Steinhart et al., J. AIDS Hum. Retrovirol. 1996; 11:271-281). Recently demonstrated ability of a mutant transcriptionally active HSF1 (lacking C-terminal residues 203-315) to suppress HIV promoter activity further suggests that HSF1 could serve as a tool for the treatment of AIDS (Ignatenko and Gerner, Exp. Cell Res. 2003; 288:1-8; see also Brenner and Wainberg, Expert Opin. Biol. Ther. 2001; 1:67-77). Since HSR1 is essential for HSF1 activation, the constitutively active form of HSR1 can mimic hyperthermia and inhibit HIV transcription.
Due to interaction of HSPs with numerous regulatory proteins (e.g., NF-κB, p53, v-Src, Raf1, Akt, steroid hormone receptors) and pathways (e.g., inhibition of c-Jun NH2-terminal kinase (JNK) activation, prevention of cytochrome c release, regulation of the apoptosome, prevention of lysosomal membrane permeabilization, prevention of caspase activation) involved in the control of cell growth, the increased production of HSPs has potent anti-apoptotic effect (Bold, et al., Surgical Oncology-Oxford 1997; 6:133-142; Jaattela, et al., Exp. Cell Res. 1999; 248:30-43; Nylandsted, et al., Ann. N. Y. Acad. Sci. 2000; 926:122-125; Pratt and Toft, Exp. Biol. Med. (Maywood) 2003; 228:111-33; Mosser and Morimoto, Oncogene 2004; 23:2907-18). Anti-apoptotic and cytoprotective activities of HSPs directly implicate them in oncogenesis (Jolly and Morimoto, J. Natl. Cancer Inst. 2000; 92:1564-72; Westerheide and Morimoto, J. Biol. Chem. 2005, 280:33097-100). Many cancer cells display deregulated expression of HSPs, whose elevated levels contribute to the resistance of cancerous cells to chemo- and radiotherapy (Ciocca and Calderwood, Cell Stress Chaperones, 2005, 10:86-103; Calderwood et al., Trends Biochem Sci., 2006, 31:164-172). Different subfamilies of HSPs including HSP70, HSP90, and HSP27 were found to be expressed at abnormal levels in various human tumors (Cardoso, et al., Ann. Oncol. 2001; 12:615-620; Kiang, et al., Mol. Cell Biochem. 2000; 204:169-178). In some cases, HSPs are expressed at cell surface in tumors, most probably serving as antigen presenting molecules in this case (Conroy, et al., Eur. J. Cancer 1998; 34:942-943). Both HSP70 and HSP90 were demonstrated to mediate cytoplasmic sequestration of p53 in cancer cells (Elledge, et al., Cancer Res. 1994; 54:3752-3757). Inactivation of wild-type p53 function has been observed in variety of cancer cells and is in fact one of the most common hallmarks in human cancer (Malkin, et al., J. Neurooncol. 2001; 51:231-243). Other studies have demonstrated that HSP70 has a potent general antiapoptotic effect protecting cells from heat shock, tumor necrosis factor, serum starvation, oxidative stress, chemotherapeutic agents (e.g., gemcitabine, torootecan, actinomycin-D, campothecin, and etoposide), and radiation (Jaattela, et al., EMBO J. 1992; 11:3507-3512; Jaattela, et al., J. Exp. Med. 1993; 177:231-236; Simon, et al., J. Clin. Invest 1995; 95:926-933; Karlseder, et al., Biochem. Biophys. Res. Commun. 1996; 220:153-159; Samali and Cotter, Exp. Cell Res. 1996; 223:163-170; Sliutz et al., Br. J. Cancer 1996; 74:172-177). At the same time, HSP70 is abundantly expressed in human malignant tumors of various origins, not only enhancing spontaneous growth of tumors, but also rendering them resistant to host defense mechanisms and therapeutic treatment (Ciocca, et al., Cancer Res. 1992; 52:3648-3654). Therefore, finding a way to suppress HSP overproduction in cancerous cells will be invaluable for increasing the efficacy of the existing anti-cancer therapeutic approaches.